Entropy 2015, 17, 2544-2555; doi:10.3390/e17042544 OPEN ACCESS entropy ISSN 1099-4300 www.mdpi.com/journal/entropy Article Thermodynamic Analysis of Double-Stage Compression Transcritical CO2 Refrigeration Cycles with an Expander Zhenying Zhang *, Lirui Tong and Xingguo Wang Institute of Architecture and Civil Engineering, North China University of Science and Technology, Tangshan 063009, China; E-Mails: [email protected] (L.T.); [email protected] (X.W.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +86-315-259-7073. Academic Editor: Kevin H. Knuth Received: 17 March 2015 / Accepted: 15 April 2015 / Published: 22 April 2015 Abstract: Four different double-compression CO2 transcritical refrigeration cycles are studied: double-compression external intercooler cycle (DCEI), double-compression external intercooler cycle with an expander (DCEIE), double-compression flash intercooler cycle (DCFI), double-compression flash intercooler cycle with an expander (DCFIE). The results showed that the optimum gas cooler pressure and optimum intermediate pressure of the flash intercooler cycles are lower than that of the external intercooler cycle. The use of an expander in the DCEI cycle leads to a decrease of the optimum gas cooler pressure and little variation of the optimum intermediate pressure. However, the replacement of the throttle valve with an expander in the DCFI cycle results in little variation of the optimal gas cooler pressure and an increase of the optimum intermediate pressure. The DCFI cycle outperforms the DCEI cycle under all the chosen operating conditions. The DCEIE cycle outperforms the DCFIE cycle when the evaporating temperature exceeds 0 °C or the gas cooler outlet temperature surpasses 35 °C. When the gas cooler exit temperature varies from 32 °C to 48 °C, the DCEI cycle, DCEIE cycle, DCFI cycle and DCFIE cycle yield averaged 4.6%, 29.2%, 12.9% and 22.3% COP improvement, respectively, over the basic cycle. Keywords: CO2 transcritical refrigeration cycle; double-stage compression; expander; coefficient of performance; intercooler Entropy 2015, 17 2545 1. Introduction Refrigerant alternatives and saving energy have become hot topics in the field of refrigeration and air conditioning. As a natural refrigerant, carbon dioxide has received increasing attention owing to its zero ODP and negligible GWP. Furthermore, carbon dioxide also has desirable thermodynamic properties, such as large specific heat, low viscosity, and large heat conductivity. However, due to the high throttling loss, the energy efficiency of the basic transcritical CO2 cycle is lower than that of the conventional low pressure refrigeration cycle. In order to improve the performance of the transcritical CO2 cycle, some modifications of the basic cycle have been tried, such as the introduction of internal heat exchangers [1–3], the use of two-stage compression systems to enhance the compression process [4,5], or an expander or an ejector to recover the expansion work [6,7]. Cecchinato et al. [5] discovered that thermodynamically the double-throttling, double-compression split cycle presents the greatest COP improvement among the different two-stage CO2 cycles. Cho et al. [8] experimentally found that the double-compression transcritical CO2 cycle with gas injection yields a 16.5% improvement of the cooling COP over that of the two-stage non-injection cycle. Cho et al. [9] numerically found that the single-stage CO2 cycle with an expander, the double-stage external intercooler cycle, and the double-stage CO2 cycle with vapor injection yield 28.3%, 13.1%, 18.3% improvements of the cooling COP over that of the basic CO2 cycle, respectively. Wang et al. [10] carried out an experiment on the two-stage compression cycle using R744 as a refrigerant, and showed that the COP of the double-stage flash intercooler cycle is 10.97% higher than that of the double-stage external intercooler cycle. Application of an expander to substitute the throttle valve is expected to enhance the cycle performance. Yang et al. [11] thermodynamically found that the employment of an expander to replace a throttle valve leads to about 33% COP improvement in the transcritical cycle. Baek et al. [12,13] developed a piston-cylinder-type expander and represented that the device can improve the cooling COP by 6.6% through the theoretical and experimental studies. Li et al. [14] experimentally found that their developed rolling-piston expander prototype can improve the COP of the system by at least 10%. Jia et al. [15] represented that a maximum COP improvement of 27.2% can be achieved by the use of the vane expander prototype. However, comprehensive investigations on the effect of expanders on the performance improvement of the transcritical CO2 double-compression cycle are very limited in the open literature. Baek et al. [16] theoretically found that the double-stage external intercooler cycle with an expander can achieve an improvement of up to 42% in COP over the basic cycle. Kohsokabe et al. [17] experimentally found that the employment of an expander in the double-stage external intercooler cycle leads to more than a 30% improvement in COP over the basic cycle. Kim et al. [18] performed a theoretical analysis of the DCEIE cycle, and showed that the improvement of COP and cooling capacity over the basic cycle was 23.5% and 8.6%, respectively. Zhili et al. [19] investigated the performance of several different expander-compressor arrangements for the double-stage external intercooler cycle, and found that the cycle with an expressor as the main compressor can yield the highest COP. But significantly less literature is available on the topic of the performance of the double-stage flash intercooler cycle with an expander. In this paper, the thermodynamic model of the double-compression CO2 cycles with and without an expander was developed. Based on this model, a steady-state simulation of the systems has been carried Entropy 2015, 17 2546 out. The CO2 thermodynamic property data is based on REFPROP [20]. The performance improvement of each of the double-compression cycle technologies was evaluated. 2. System Description and Analysis The schematic diagram and corresponding pressure enthalpy diagram of the base transcritical CO2 refrigeration cycle are illustrated in Figure 1. The base cycle is composed of four basic processes: compression (1–2), heat rejection (2–3), expansion (3–4) and heat absorption (4–1). Figure 1. Schematic and p-h diagram of the base cycle. In order to improve the performance of the transcritical CO2 refrigeration cycles, the following double-stage compression cycles are investigated: (1) Double-Compression External Intercooler (DCEI) cycle: the schematic diagram and corresponding pressure enthalpy diagram of this cycle are illustrated by the continuous line in Figure 2. This cycle consists of an evaporator, two compressors (LS compressor and HS compressor) with inter-cooling of the vapor at the intermediate pressure through a heat exchanger, a gas cooler, and a throttling valve. Figure 2. Schematic and p-h diagram of DCEI cycles with and without the expander. Entropy 2015, 17 2547 (2) Double-Compression External Intercooler cycle with an Expander (DCEIE): This cycle is similar to the DCEI cycle except with the expander replacing the throttle valve and using the recovery work to drive the compressor. The expansion process of the expander is illustrated by the broken line in this cycle, as shown in Figure 2. The recovered power of the expander is used to offset the work of the low-stage compressor. (3) Double-Compression Flash Intercooler cycle (DCFI): the cycle is represented by the continuous line in Figure 3. The high pressure refrigerant after the gas cooler is divided into two streams: one of them is throttled down to the intermediate pressure through the throttle valve A, and flows into the flash chamber. The entered refrigerant flashes into vapor, cools the residual stream of high pressure gas, mixes and exchanges heat with the discharged high temperature refrigerant from the LS compressor. Then the resulting mixed saturated vapor is sucked by the HP compressor. The cooled high pressure gas is expanded in the throttle valve B and then fed to the evaporator. Figure 3. Schematic and p-h diagram of DCFI cycles with and without expander. (4) Double-Compression Flash Intercooler cycle with an Expander (DCFIE): This cycle is similar to the DCFI cycle except for substituting the throttle valve B with an expander and using the recovery work to drive the compressor. The expander expansion process of the DCFIE cycle is expressed by the broken line in Figure 3. The recovered work of the expander is transmitted to the low-stage compressor. The cycles were theoretically investigated using an original simulation code under the following assumptions: • Steady-state operation. • No pressure losses in pipes and heat exchangers. • Saturated vapor at the evaporator outlet. • The compressor and the expander are treated adiabatically. The isentropic efficiency of the compressor is calculated by [21]: p η= − com, out com 1.003 0.121 (1) pcom, in Entropy 2015, 17 2548 The coefficient of performance of the DCEI cycle can be written as: h − h COP = 1 6 (h − h ) + (h − h ) (2) 2 1 4 3 For the DCEIE cycle, it can be expressed as: − h1 h ' COP = 6 (3) (h − h ) + (h − h ) − (h − h ) 2 1 4 3 5 6' For the DCFI cycle and the DCFIE cycle, the energy balance equation for the flash intercooler can be written as follows: m (h − h ) + m (h − h ) = (m − m )(h − h ) 1 2 3 1 5 7 2 1 3 6 (4) where m1 is the mass flow rate of the low stage compressor, m2 is the mass flow rate of the high stage compressor. The coefficient of performance of the DCFI cycle can be written as: m (h − h ) COP = 1 1 8 m (h − h ) + m (h − h ) (5) 1 2 1 2 4 3 For the DCFIE cycle, it can be expressed as: − m (h h ' ) COP = 1 1 8 − + − − − (6) m1(h2 h1) m2 (h4 h3 ) m1(h7 h8' ) 3.